Bert Trautmann playing for Manchester City against Wolverhampton Wanderers in 1951.
Photograph: Action Images / Reuters

Film director Marcus H Rosenmüller looks out of the car window, a little spooked. “It is only one kilometre from the concentration camp,” he says. We are in a quiet, pretty, well-heeled town, a short drive from Munich. It is called Dachau.

“It’s quite shocking to be coming to Dachau, isn’t it?” says the British producer Chris Curling. Both men agree there is something eerily appropriate about filming here. They are making a movie about the life of the legendary German Manchester City goalkeeper Bert Trautmann, who is still best known for his part in the 1956 FA Cup Final against Birmingham City. With 17 minutes of the match left, he dived at the feet of Birmingham City’s Peter Murphy, and sustained a nasty neck injury. But he continued to play, making two crucial saves as Manchester City won 3-1. Trautmann was a hero, particularly when, three days later, it was discovered that he had actually broken his neck.

Although Trautmann never played for Germany (he was not allowed to because he played his club football in England), he was already regarded as a world-class keeper (two days before the 1956 final, the Football Writers’ Association named him footballer of the year, the first goalie to win the award). After breaking his neck, he recovered and went on to play for City for another eight years. By the time he retired in 1964, Trautmann had played 545 matches for the club.

But Trautmann’s story is far more remarkable than his broken neck. He joined Manchester City in 1949, only four years after the end of the second world war. Hostility towards Germans was, understandably, still high. When I was growing up as a Manchester City fan, learning about Trautmann was a rite of passage. It was a good decade since he had retired, but he still represented everything that was great about City: bravery beyond the call of duty, tolerance, the new postwar world in which the German god in goal was embraced by one and all as “our Bert”. And yet, so close to the war, we could take our tolerance only so far. Bert, we were told, was different from the Nazis who had made it their mission to wipe out the Jews and the Slavs and the Gypsies and the gays; he was the good German.

But the reality was very different. In fact, Trautmann was a high-achieving Nazi who fought on the eastern front, embraced Hitler wholeheartedly, and was awarded the Iron Cross 1st Class for bravery in battle. And it is this that makes the Trautmann story even more amazing than I was led to believe as a child.

The production is on location at a big old house in Dachau that serves as the hospital where Trautmann discovers he has broken his neck. It is summer and the actor John Henshaw is wandering around in a three-piece tweed suit. He is a big man, not entirely comfortable in the heat. “It was 36C last week, and I had this on and a big overcoat on top of it. I was sweating like a cheese,” he says. Henshaw plays Jack Friar, the club secretary who signed Trautmann for St Helens (his club before City), and eventually became his father-in-law.

Trautmann leaving the pitch after the final in 1956 with what he didn’t know was a broken neck until several days later. Photograph: Popperfoto

Henshaw talks of the hostility towards Trautmann when he signed for City. “When he first started playing, he was getting death threats. God, what a brave man he must have been!”

The Manchester public had good reason to be suspicious of Trautmann. City has always had a large Jewish following, and many felt betrayed by the club when they signed the goalkeeper. An estimated 20,000 fans stood outside the Maine Road stadium shouting “Nazi” and “war criminal” and threatening to boycott the club. The film-makers have recreated newspapers from the time with headlines such as “Send the Nazi home” and “Kraut goalkeeper with Iron Cross,” and “Man City’s goalkeeper doesn’t want to remember our pain”. One article quotes him as saying: “I did what all soldiers do. I had no choice!”

Trautmann had been a tough, sporty boy, with little time for vulnerability. He despised his father’s weakness for drink and compromise, and venerated the Führer for rebuilding the economy, championing sport and marshalling a master race. He joined the Hitler Youth, and – aged 17 – volunteered for the army. In the early days, at least, Trautmann revelled in the war – and in the cause. Where better for a testosterone-fuelled Aryan idealist to express himself? In the book Trautmann’s Journey, written by Catrine Clay in collaboration with its subject, we learn that the young Bernhard was a model Nazi: blond, blue-eyed, bigoted and ruthless. In one memorable passage, he describes how he would simultaneously stock up on cigarettes and entertain himself during downtime in the war. This would involve going into town, beating up Italian soldiers (they might have been fighting on the same side, but he despised their weakness) and relieving them of their cigarettes.

On having witnessed a mass extermination, Trautmann said: 'If I’d been a bit older, I’d probably have committed suicide'

By all rights, he should have been dead before he even discovered his gift for goalkeeping. On the Russian front, as the Nazi forces retreated, Trautmann was blown up but survived. In France, he was buried in rubble for three days after being bombed again. He was captured by the Russians and the French but escaped both times. In 1944, he was one of the few survivors of the Allied bombing of Kleve, and was trying to get home to Bremen when he was caught by two American soldiers in a barn in France. The soldiers decided Trautmann had no useful information to give them so marched him out of the barn with his hands held up. He thought he was going to be shot, so he fled, jumping over a fence. However, he landed at the feet of a British soldier, who greeted him with the words: “Hello Fritz, fancy a cup of tea?” This time, he didn’t run.

Trautmann was one of only 90 survivors from a regiment of 1,000 men. He became a prisoner of British forces and ended up in a PoW camp in Lancashire. At the camp, he played as a goalkeeper for the first time. He had been a centre half, but got injured in one match and typically refused to go off so they stuck him between the sticks, where he remained for ever after. There were three categories of prisoner at the camp: white for anti-Nazis, grey for unsure and black for unrepentant Nazis. Trautmann was one of around 10% classified as Nazis. All this, and he was still only 22. No wonder Rosenmüller thought he was a rich subject for a film.

David Kross as Trautmann in The Keeper. Photograph: Aidan Monaghan/Zephyr Films Trutmann Ltd

The German actor David Kross, who starred in the Oscar-winning film The Reader, is sitting in his goalkeeper’s kit reading Trautmann’s biography, waiting to be called. “He has got a very polite, friendly mask,” says Kross of Trautmann. “But he can get really angry.” Has he played anybody as angry before? “Not really, no. It is a big task for me. I am normally nice!” When I ask Kross what drives the goalkeeper, he immediately says guilt. “He’s seen some terrible things. It’s the guilt of not acting against it or not doing something to stop it.” Trautmann and fellow paratrooper Peter Kularz witnessed a mass extermination in a forest: men, women and children were herded into a trench and shot by an Einsatzgruppen, a Nazi paramilitary death squad. Trautmann and Kularz crawled away on their bellies, then ran for their lives. If they had been spotted, they would have been shot on the spot because the Einsatzgruppen wanted no witnesses. Years later, Trautmann admitted he was still haunted by what he saw. (“If I’d been a bit older, I’d probably have committed suicide,” he said.)

It is the mix of guilt and anger that makes Trautmann fascinating. He had none of the humility you might expect from a man tortured by what he did and saw. As a prisoner of war, he could not understand why a Jewish officer might be abusive to him. “He had to drive the Jewish officer to different locations,” Kross says. “And the Jewish officer treated him badly, so he hit him. And yet this complex, contrary man unwittingly became a messenger of peace.”

The astonishing thing is that so little is known about Trautmann in Germany. Kross admits he had never heard of him before being offered the part. The actor hopes that the film, which is largely funded with European money, will make the keeper famous in his home country.

Rosenmüller, a Bavarian film-maker who played semi-professional football himself, has been working on the project for almost a decade. He was making a film with the producer Robert Marciniak, who told him about the German goalkeeper who broke his neck playing for an English team in the FA Cup final. Both men became obsessed with Trautmann – and turning his life into a movie. They first met him in Nuremberg, when he received a medal from the German FA. They then spent a week with him in 2010 at his home near Valencia in Spain (Trautmann died in 2013).

Maine Road legend … Trautmann at the old Man City ground. Photograph: ANL/REX/Shutterstock

Rosenmüller says he was a closed man who gradually opened up. “He first showed us a bit of himself, and then more. He told us he had not had enough courage to make things different in the war.” I ask Rosenmüller if he feels Trautmann was being honest – did he feel bad because he did not have the courage to stop things happening, or did he feel bad because of the things he did himself? Probably both, Rosenmüller says. “He went as a volunteer, and I’m sure he wanted to be a good soldier.”

Even if he never fully addressed his war crimes, Rosenmüller says to go as far as he did was remarkable. Trautmann belonged to a generation that found it almost impossible to talk about what they had seen, whether as victims or as perpetrators. “He was honest to say he could not interfere because he lacked courage. It was not like: “I couldn’t do anything.’ It was: ‘Shit, I didn’t have the courage.’ For me, it was heroic to say: ‘I did wrong.’”

There are lots of heroes in the Trautmann story – the Friar family, Manchester City football club, the supporters. But perhaps the biggest hero is Rabbi Alexander Altmann, whose parents were killed in the Holocaust. In an open letter to the Manchester Evening Chronicle, he wrote that Trautmann should not be punished “for the terrible cruelties we suffered at the hands of the Germans ... If this footballer is a decent fellow, I would say there is no harm in it. Each case must be judged on its own merits.” After Altmann’s letter, the protests stopped. Trautmann went on to win an OBE for his work for Anglo-German relations.

For Rosenmüller, the story is a personal one. He has always wondered how he would have turned out if he had joined the Hitler Youth and been indoctrinated by nazism. Would he have had the strength to resist, or would he, like Trautmann, have volunteered for the army? But most of all, he says, it is the theme of reconciliation that inspires him. “I once made a documentary about a farmer because the farmer was fascinated with Bishop Tutu. Tutu’s Truth And Reconcilation Commission gave South Africans the opportunity to admit they had done terrible things, and people forgave them. That was such an incredible thing. And it’s a pity that we Germans never did that.” Rosenmüller says that the failure of the Nazi generation to address their crimes led to the student protest movement of 1968 – a generation of German youth revolting against their parents’ silence.

And yet in Lancashire in the late 1940s, a formerly unquestioning Nazi loyalist was interned in a prisoner of war camp, and forced to address his crimes and prejudices. And the community, inspired by the words of Altmann, did forgive him. As far as Rosenmüller is concerned, that is the heart of his film. Yes, Trautmann was a great goalkeeper. Yes, he was ludicrously brave. But more important than anything, he became the living embodiment of truth and reconciliation.